Various processing techniques have heretofore been proposed or used for manufacturing turbine wheels for use in gas turbine power plants of the types satisfactory for use in automobiles, buses, trucks, off-the-road earthmoving equipment, and the like. The manufacturing techniques employed in the fabrication of aircraft type gas turbine engines are exceedingly costly and are not applicable for the mass production of low-cost vehicle turbine power plants that are competitively priced with gasoline and deisel-fueled reciprocating internal combustion engines. The various prior art processing techniques heretofore proposed for fabricating gas turbine wheels have either been too costly or have produced turbine wheels which are of inadequate mechanical strength and/or durability, or are not adaptable to high volume mass production techniques.
As an example, it has heretofore been proposed to produce such turbine wheels as an integral casting of a high strength, high temperature alloy employing investment casting techniques. Unfortunately, the rate of solidification of such integrally-cast wheels results in an extremely fine-sized grain structure in the blade portions of the wheel and a macro-sized grain structure in the central portion of the hub or rotor which imparts brittleness and lack of toughness and strength in the hub portion, which during service is subjected to the highest stresses due to the centrifugal forces imposed on the turbine wheel. The fine-sized grain structure of the blades, on the other hand, prevents the attainment of optimum high temperature strength and fatigue resistance properties. In order to overcome the aforementioned problem, it has been suggested, such as in U.S. Pat. No. 2,894,318, to form composite turbine wheels by separately forming the blade elements and thereafter casting and integrally bonding the blade elements to a cast central hub or rotor. Alternatively, it has been suggested, such as in U.S. Pat. No. 3,032,864, to produce a composite turbine wheel by mechanically bonding a plurality of separately-formed blade elements to a separately-formed central hub section by a high temperature forging operation in which the hub section is deformed into mechanical interlocking relationship with the blade elements.
The foregoing and other prior art techniques heretofore proposed have been unsatisfactory for any one of a number of reasons and the process of the present invention overcomes the problems and disadvantages heretofore encountered, enabling the manufacture of composite turbine wheels of satisfactory strength and durability at commercially acceptable costs employing mass production techniques.